While you may know where your check engine light is (especially if it's currently illuminated on your dashboard), we understand that most drivers do not know why it’s on. As soon as your check engine light comes on, you should consider finding the check engine light code to determine what exactly is going on.

Today, we’ll teach you how to find check engine codes, help you understand what they mean, and offer guidance on what to do with that information.

How Do I Find Check Engine Codes?

Really, there are two options. One, you can visit an auto mechanic and have them diagnose your check engine light. Two, you can purchase an engine code reader for personal use and pull the codes yourself. An engine code reader can be used to identify why your check engine light came on.

If you have no idea how to decode what fault code is causing your checking engine light, don't lose hope! You don't have to be a professional auto mechanic to figure it out, but you should know that the reader won't lay out your engine's problem in plain English. It will give you a code that starts with a letter and is followed by four numbers. Let’s talk about those letters and numbers.

What Do Check Engine Codes Mean?

There are four different on-board diagnostics, or OBD, fault codes that'll indicate what may need to be repaired or serviced in your vehicle:

• P – Powertrain

• B – Body

• C – Chassis

• U – Network Communications

The numbers following the letter offer some clarity by narrowing down where the problem lies.

The Most Frequent Diagnostic Codes for Check Engine Lights & Their Meanings

Some of the most common check engine codes are powertrain related. There are a lot of codes, so for today’s blog we’ll cover 5 of the more common ones.

1. Engine Misfire: P0300 to P0305

Getting P0300, P0301, P0304, or P305 from a scan means your engine has misfired. In a combustion engine, the combination of spark and fuel result in a tiny explosion; if something prevents that tiny explosion, then the cylinder will remain unfired. The number in the code indicates which cylinder failed to fire. These misfires can be traced back to any number of reasons, ranging from a problem with the spark plugs or plug wires to the distributor cap coils of the fuel injectors.

2. Emissions Issues: P0400 to P0499

Your check engine light may have come on to indicate a problem with your evaporative emissions system, which prohibits harmful vapors from escaping from your vehicle and entering into the atmosphere. The failure can arise from a variety of reasons, including a loose or defective gas cap, a broken gas cap seal, or a leak in a plastic tube that runs between the engine compartment and the fuel tank.

Emissions Code P0456 is the most common code for emissions issues and implies simply that there is a small leak in the system somewhere. It must be remedied before attempting to pass an emissions test.

3. Failed Oxygen Sensor: P0150 to P0175

Any codes between P0150 to P0175 could mean a problem with your engine's oxygen sensor. Your car's oxygen sensor (O2 sensor) measures and regulates the amount of unburned oxygen in your vehicle's exhaust system.

Faulty sensors can damage your car's spark plugs and catalytic converter and cause your car to burn more fuel than necessary. The code can tell you whether your problem is with the senior itself or the intake is either too rich or too lean. Rich? Lean? What do these words mean in vehicle jargon?

Rich and Lean refer to the necessary ratios of fuel and air. If an engine has too much fuel and not enough air, it is running rich. You can remember it this way - gasoline can be expensive or “rich”, whereas breathing air is free. If an engine has too much air and not enough fuel, it is running lean.

4. Catalytic Converter Problems: P0420 to P0434

Your catalytic converter receives harmful carbon monoxide and “converts” it into carbon dioxide and is vital to your car's air quality. A code between P0420 and P0434 may be thrown out if something is wrong. Faulty oxygen sensors, damaged spark plugs, and bad wires may be the cause of your catalytic converter fault code and will need to be immediately repaired or replaced to ensure your car can be driven again.

5. Mass Airflow Sensor Damage: P0101 to P0104 & P010A to P010D

Your mass airflow system (MAF) measures the amount of air entering your car's engine and determines how much fuel your engine requires to run efficiently.

A damaged mass airflow system can reduce your vehicle's performance and result in low fuel economy, as well as damage or ruin your spark plugs. The specified number or letter will inform you of which exact circuit has been damaged.

I Diagnosed My Check Engine Light – Now What?

Once you have your code in hand and properly diagnose the problem, you'll be able to provide clearer information to your mechanic when you bring your vehicle in for service. They'll be able to double-check the code and complete a full inspection to highlight any other areas of concern and get them repaired before another light pops on. Even if you haven't figured out what your code means, bring your car to the local mechanic for help!

You might be wondering, “If I’m going to take my car to the mechanic anyway, then why spend the time buying a code reader and self diagnosing my car?” That’s actually a great question, and we feel this is the perfect time to answer it. Can you self diagnose car problems? Sometimes, yes. Often, however, a code reader is like a street sign, telling you which road the problem is on but not the house where the problem lives. Many times codes offer a direction, but a skilled technician has to test many components to determine which part is actually faulty. That being said, do your best on your own, and if you end up at a dead end, call Christian Brothers Automotive!

Get Help from a Trusted Automotive Repair Company

Be sure to seek out the assistance of an honest, reputable auto mechanic, or contact Christian Brothers Automotive! Our team will be with you every step of the way, ensuring you have all the help you need to get your check engine light diagnosed, problem fixed, and your car back on the road in no time.

Contact your closest Christian Brothers Automotive shop to learn more about our dedication to service or to schedule your visit! We are about fixing cars, but we’re also about Driving Joy!

This blog was written January, 2013, and was updated September, 2023, to reflect current trends and best practices.